The invention relates generally to a cranks haft counterweight for an internal combustion engine.
A crankshaft for an internal combustion engine typically comprises a shaft having at least one eccentric crankpin. A rod connects the engine""s piston to the crankpin so that linear movement of the piston is translated to rotation of the main shaft. In order to balance rotation an eccentric mass is attached to the shaft to counterbalance the mass of the crankpin, rod, and the piston. The cranks haft can either have double counterweights or be cantilevered. With a double counterweight cranks haft, force from the piston is applied at a point, or at points that distribute the force evenly between the bearings supporting the shaft. Typically, a balanced cranks haft has, for each piston, two closely spaced counterweights, joined by a crank pin to which a rod is connected. Each counterweight is attached to a shaft. A cantilevered cranks haft has only one counterweight and shaft.
For two-cycle engines, it is common to use a xe2x80x9cTxe2x80x9d shaped counterweight. A crank pin is attached near the bottom of the stem of xe2x80x9cTxe2x80x9d. One shaft (in the case of a cantilevered cranks haft) or two shafts (in the case of a balanced cranks haft) are attached to the counterweight(s) at its dimensional center. A xe2x80x9cTxe2x80x9d shaped counterweight is not strictly or literally in the form of a xe2x80x9cTxe2x80x9d. Rather, the xe2x80x9cTxe2x80x9d description suggests a relative distribution of mass to a side opposite of the crank pin and perpendicular to its axis of rotation. Shapes that are closer to that of a triangle or a triangle with two squeezed sides are also considered xe2x80x9cTxe2x80x9d shaped counterweights. xe2x80x9cTxe2x80x9d shaped counterweights are typically made from a single piece of metal, but have also been constructed using xe2x80x9claminationsxe2x80x9d or a multiple, stacked plates cut in a xe2x80x9cTxe2x80x9d shape and joined by, for example, pins. Examples of these types of counterweights are shown in U.S. Pat. No. 4,342,236. Most of the mass of a xe2x80x9cTxe2x80x9d shaped counterweight is concentrated where it is most effective, while reducing the overall mass of the counterweight.
To transfer a charge of a fuel and air mixture from a carburetor into a cylinder, a process referred to as scavenging, two-cycle engines typically draw into the engine""s crankcase the charge using a pressure decrease generated by upstroke of the piston. The down stroke of the piston then compresses, and thereby pressurizes, the charge prior to an intake port opening on the cylinder. The relatively higher pressure of the charge causes it to flow into the relative lower pressure cylinder. Increasing the pressure differential enhances scavenging of the cylinder. Better scavenging tends to improve engine performance and reduce emissions. Because the volume that the piston displaces during movement is fixed, decreasing the volume of voids within the crankcase will tend to increase pressure within the crankcase and thus improve scavenging.
Some of the volume of the voids within the crankcase is necessary to accommodate the sweep of the counterweight. Because the swept volume of the counterweight in particular xe2x80x9cTxe2x80x9d shaped counterweights is significantly greater than the actual volume of the counterweight, it is desirable to fill this void. One prior art approach is to extend the shape of the counterweight to form a full circle. Most of circle is relatively thin, but it includes a much thicker, xe2x80x9cTxe2x80x9d-shaped portion forming the counterweight and support for attaching a crank pin and cranks haft. A drawn metal cup having a circular shape is attached to the edges of the extended counterweight to enclose voids on either side of the thick xe2x80x9cTxe2x80x9d shape and the extended edge.
This example of a xe2x80x9cfull circlexe2x80x9d counterweight has several problems. First, the counterweight is made of forged steel in order to make it into a circular shape with most of its mass concentrated in the thick xe2x80x9cTxe2x80x9d-shaped area. Forging the counterweight is a relatively expensive process. Second, because the steel extends to the edges of the circle, the overall mass of the counterweight is increased. Further, the mass is added in places where it is not useful for counterbalance. Not only is the additional weight an extra load on the engine, it is also detrimental to balance of the cranks haft.
Another example, useful for laminate-constructed counterweights, is to cut openings in each of the inner layers of the laminate to form in each layer a xe2x80x9cTxe2x80x9d shape surrounded by a circle, and attach complete, circular shaped layers on the outside to seal the voids created by the openings in the inner layers on the outer layers, thus creating sealed voids. However, manufacture and assembly of this type of full circle counterweight is relatively complicated, and extra mass is added in places where it is not useful for counterbalance.
The invention relates to an improved full circle counterweight for a cranks haft of an internal combustion engine that overcomes one or more of the problems with prior art.
The invention, as defined by the claims, is disclosed below as part of a detailed description of an example of two-cycle engine for a power tool having a full-circle counterweight assembly.
The full circle counterweight assembly in the disclosed example includes a counterweight that is shaped generally like that of a conventional xe2x80x9cTxe2x80x9d-shaped counterweight. Like a conventional xe2x80x9cTxe2x80x9d shaped counterweight, it includes openings for receiving a crank pin and a shaft. The T-shaped metal counterweight is partially encased within a retainer in the form of a drawn metal cup. Two lightweight spacers, made for example from plastic, fill voids on opposite sides of the stem of the xe2x80x9cTxe2x80x9d shaped counterweight. Edges of the cup are folded over to retain the inserts within the cup, and to retain the cup on the metal counterweight. As compared to the prior art, little additional weight is added to make the counterweight full circle. Further, as compared to prior art, the full circle counterweight assembly tends to be more easily and less expensively made. The metal counterweight, cup and lightweight inserts can be assembled according to standard, relatively low cost methods. The assembly is relatively simple, as it involves relatively few parts and steps. The full circle counterweight finds particular advantage in two-cycle engines of the type used in power tools and equipment, as such engines must be both lightweight and easily manufactured to reduce cost, while meeting increasingly stringent emission requirements.